Radio navigation beacon including antenna element commutation

ABSTRACT

A radio navigation beacon system of the Doppler type having a simplified antenna arrangement. A main array employing 30 array elements spaced at four wavelengths and an auxiliary or reference array comprising 12 reference antenna elements spaced at onethird wavelength, are employed. A source of radio frequency energy is commutated to the separate radiator elements in order to simulate unidirectional or bidirectional constant velocity motion of the source. Navigational information is derived from this ground beacon arrangement by a receiving station as, for example, in an approaching aircraft on the basis that the Doppler shift of frequency observed at the receiving station is proportional to the cosine of the angle of reception with respect to the operating centerline of the system. Means are included for commutating the reference frequency to each reference antenna element, in turn, during the period that each main array element is radiating.

ite States Patent 1 Overbury 51 Apr. 17, 1973 I RADIO NAVIGATION BEACONINCLUDING ANTENNA ELEMENT [21] Appl. No.: 210,699

US. Cl. ..343/l08, 343/106, 343/854 Int. Cl. ..G0ls l/l6 Field of Search..343/l06 D, 108 M,

[56] References Cited UNITED STATES PATENTS 3,670,337 6/1972 Earp et a1...343/l06 D Primary Examiner-Benjamin A. Borchelt isskrtggt Examiner-A.M. Psitos 5 7 ABSTRACT A radio navigation beacon system of the Dopplertype having a simplified antenna arrangement. A main array employing 3Oarray elements spaced at four wavelengths and an auxiliary or referencearray comprising 12 reference antenna elements spaced at onethirdwavelength, are employed. A source of radio frequency energy iscommutated to the separate radia tor elements in order to simulateunidirectional or bidirectional constant velocity motion of the source,Navigational information is derived from this ground beacon arrangementby a receiving station as, for example, in an approaching aircraft onthe basis that the Doppler shift of frequency observed at the receivingstation is proportional to the cosine of the angle of reception withrespect to the operating centerline of the system. Means are includedfor commutating the reference frequency to each reference antennaelement, in turn, during the period that each main array element isradiating.

10 Claims, 1 Drawing Figure 512 r at A 4 r H l [Mina/M5 fommz/Zazorfam/nuzazor C2 /5z 200 RI freq. #2 Freq.

Sou/ ce Sou/ c fxc/ten RADIO NAVIGATION BEACON INCLUDING ANTENNA ELEMENTCOMMUTATION BACKGROUND OF THE INVENTION 1 Field of the Invention Thisinvention relates to radio navigation beacons and, more particularly, toinstrument landing systems providing for angle determination withrespect to an ideal course or glide path for an aircraft during landingapproach.

2. Description of the Prior Art The present invention constitutes animprovement over basic Doppler instrument landing system arrangementsdescribed in US. Pat. application Ser. No. 859,915, filed 22 Sept. 1969,now US. Pat. No. 3,626,419, and U.S. Pat. application Ser. No. 4,653,filed 21 Jan. 1970, now U.S. Pat. No. 3,670.337. Those patentapplications are assigned to the assignee of the present invention.

The aforementioned prior art describes a basic system with variationsusing linear array radio beacons, in which a source of radio frequencyenergy is commutated to separate radiator elements in order to simulateunidirectional or bidirectional constant velocity motion of the source.Navigational information is derived from these beacons, as for example,by an approaching aircraft in a landing pattern, on the basis that theDoppler shift of the received radio frequency is proportional to thecosine of the angle which the remote radio receiver subtends withrespect to the line of movement (bore sight or centerline) of the source(ground beacon antenna array).

As movement is simulated by the successive commutation of the source toadjacent elements of such a Doppler array, the spacing of these elementsis determined by the limiting phase transient which can be toleratedbetween successive samples of received signal. In general this transientcorresponds to a phase jump of 120". In the case where information isrequired over a wide angle, this may require adjacent elements in anarray to be spaced as close as one-third wavelength.

The accuracy and integrity of such a Doppler navigation system isdetermined mainly by the array length and, in the case of systemsrequired to furnish angular information to an accuracy of 0.01 degrees,an array length of as much as 120 wavelengths could be required. Therealization of a Doppler navigation system to such an accuracyrequirement over a wide sector would therefore involvev the use of 360radiator elements. The use of a Doppler array beacon of that size, withits associated commutator and cables, constitutes a prolixity ofequipment and introduces possible RF losses detracting from thesimplicity of the basic system.

Typically, the linear arrays of the prior art and present systems areoriented so that their radiating elements are distributed along a linenormal to the ideal flight path line (for the application of theinvention to aircraft final landing approach navigation). Used for theazimuth navigational aspect of such navigation (localizer function), thearray would extend horizontally in a line normal to and across therunway centerline. For the elevation (glide path) application, the arraywould be disposed generally vertically or tilted from vertical by asmall angle corresponding to the glidepath angle (measured from thehorizontal plane). That glide-path angle is typically on the order of 2or 3 for aircraft (except VTOL types).

More information as to actual array placements is to be understood fromthe prior art aforementioned, however, it is useful to bear the physicalrelationships in mind in understanding the present invention. Thus, inthe localizer instrumentation, the approaching aircraft receives energysuccessively from the commutated elements of the primary array over achanging path length. The said path length is minimum when the aircraftis directly on course during the time it receives energy from the middleelement of the primary array. The end elements of the primary arrayprovide the longest path length and throughout the commutation of theprimary array the aircraft receives a Doppler component which variesthrough a point of inflection at the time of reception from the centerelement. Sense information is provided by the reference array in amanner also explained in the aforementioned prior art.

For purposes of the description hereinafter, the expression idealnavigational path refers to either the azimuth or elevation situationand means the on-course line (runway centerline) or correct glide pathline,

respectively.

The Doppler navigation system discussed above typically operates at aradio frequency of at least 1.0 GI-lz, and since the maximum Dopplerfrequency shift involved is of the order of a few KHz, it is necessaryin practice to use a reference antenna at the beacon, which radiates asecond radio frequency slightly offset from the commutated frequency,e.g. by 20 KHz. The Doppler shift of the frequency of the movingcomponent is then detected as a change on the beat frequency between themoving component and the reference signal. Thus, the indicated change ofbeat frequency, which bears the navigational information, is determinedby the change of path difference between the two paths. With the fixedreference antenna of the above described prior art system, this changeof path difference arises solely from the movement simulated by thecommutated array.

In order to simplify the overall system, improvements were devised whichare hereinafter described.

SUMMARY OF THE INVENTION According to the invention, there is provided aradio navigation beacon including a first linear array of equally spacedradiating elements, an RF energy commutator provides the RF switching ofthe first radio frequency signal to each said radiator in turn. A secondlinear array of equally spaced elements in substantial alignment withthe first array has an overall length equal to the spacing betweenadjacent elements of the first array. Another energy commutator switches(commutates) energy of a second radio frequency signal (different fromsaid first radio frequency signal) to each of the second array elementsin succession during the'period of excitation of each of the first arrayelements by the first radio frequency. Commutation to the two arrays isin opposite directions.

BRIEF DESCRIPTION OF DRAWINGS A single FIGURE depicts the system of theinvention in block diagram form.

DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the inventionwill now be described with reference to the single FlGURE'of the Dopplerarray having a length of 120 wavelengths (120 A). Instead of 360 priorart elements spaced A: A"

between adjacent elements, the present array (hereinafter termed primaryarray") consists of 30 elements, A, with spacing S1 of 4 A betweenadjacent elements, i.e., the primary array has an overall length ofInstead of the prior art single fixed reference antenna, there are 12reference radiator elements, B, arranged as a linear array hereinaftertermed reference array"), with aspacing S2 of A between adjacentradiator elements, i.e., the reference array has an overall length of 4A.

In general, therefore, the length of the reference array is equal to thespacing between adjacent elements of the primary array, and the spacingbetween adjacent elements of the reference array is equal to the'stepchange of path length required.

The elements of the primary array and the reference array areconveniently mounted in alignment on a common support member M.

The required change of path length (commutation program), generated as360 steps each of A: )t, is achieved as follows.

A first radio frequency (f) from a first source R1 is commutated in turnby a commutator C1 to the elements of the primary array at a scanningrate determining the apparent velocity ofa moving source.

During the time each element of the primary array is radiating at thisfrequency, a second, slightly offset reference radio frequency from asecond source R2 is commutated in turn by a commutator C2 (in theopposite direction of scan to that of the primary array) to the elementsof the reference array.

The drawing also includes an initial radio frequency source or common RFexciter R3 from which are derived, as indicated by the interconnectinglines, both the array frequencies and also the appropriate controllingfrequencies to controllers P1 and P2 for the respective commutators.Since these controller functions are merely timed switching, as will beevident hereinafter, their detailed instrumentation will be readilyassembled by those skilled in this art.

Identifying the successive elements of the primary array, in thedirection of scanning of the primary array, as Al, A2, A3, et cetera,and the successive elements of the reference array, in the direction ofscanning of the reference array, as B1, B2, B3, et cetera, the sequencefora single scan is A1, B1; ALBZ', A1. B3; ALBIZ:

A12, B1; A2, B2; -A2, B3; ..Al2,B1l2;

A30, 81; A3082; A30, B3; ..A30.Bl2.

This sequence is then reversed, for bidirectional scanning, or repeatedfor unidirectional scanning. As

explained in the above-mentioned Specifications, the reference frequencymay be less than f or greater than f with unidirectional scanning, ormay alternate between f and f according to the direction of scan of theprimary array with bidirectional scanning.

This sequence results in the required 360 steps, with the 1% A stepsbeing derived as a result of the opposite simulated movement of theelements of the reference array.

ln this way, the number of incremental steps available is equal to theproduct of the number of reference elements and primary elements. In thedescribed embodiment there is a total of 42 antenna radiatorelements, asopposed to the 361 elements requiredin the prior art version.

By suitably choosing the spacing of the elements of the primary arrayand of the reference array, together with suitable scanning rates foreach array, other commutation sequences are possible .in addition to theforegoing technique which may be termed step-andrepeat".

. It is to be understood that the foregoing description of specificexamples of this invention is made by way of example only and is not tobe considered as a limitation on its scope.

What is claimed is 1. In a radio navigation beacon system whichsimulates motion by electrical sequencing of antenna elements togenerate an apparent-Doppler frequency component at a receiving station,the combination comprismg: i I

a first source of radio frequency energy of a first frequency; I

a first antenna array. including a plurality of equally spaced antennaradiating elements disposed in a line at an angle with respect to theideal navigational path;

first commutating means-forsuccessively switching the output of saidfirst-source to each of said radiating elements of said first arrayaccording to a predetermined pattern;

second antenna array including a plurality of equally spaced antennaradiating elements disposed in a line at an angle with respect to saidideal navigational path, said second array having an overall lengthequal to the spacing between adjacent elements of said first array; asecond source of radio frequency energy of a second frequency differentfrom said frequency; and second commutating means for successivelyswitching the output of said second source of radio frequency energy toeach of said radiating elements of said second array in successionduring the time of energization of each of said first array elements bysaid energy of said first frequency.

2. Apparatus according to claim 1 in which the elements of said firstand second arrays are disposed substantially on the same line.

3. Apparatus according to claim 2 in which said second commutating meansis adapted to switch said energy of said second frequency successivelyamong said elements of said second array in the opposite direction alongsaid same line as the direction of switching provided by said firstcommutating means at any time.

4. Apparatus according to claim 3 in which said angle of said same linealong which said first and second arrays are disposed makes an angle ofsubstantially 90 with respect to said ideal navigational path.

5. Apparatus according to claim 3 in which said first array isidentified as the primary array and said second array is identified asthe reference array, in which said primary array is defined as having anoverall length which is an integral number of wavelength times theoverall length of said reference array.

6. Apparatus according to claim 5 in which the elements of said primaryarray are spaced four wavelengths apart and the elements of saidreference array are space one-third wavelength apart, whereby said firstand second commutating means operate to produce discrete radiated energypath length changes in steps of one-third wavelength.

7. Apparatus according to claim 6 in which said primary array comprises30 elements and said reference array comprises 12 elements.

8. Apparatus according to claim 5 in which said second frequency isdefined as being different from said first frequency by an amountcomparable to the Doppler frequencies produced by said commutators, saiddifference between said first and second frequencies being a smallpercentage of said first or second frequency.

9. Apparatus according to claim 8 in which said frequency. difference ison the order of 20 KHz, whereas said first and second frequencies are onthe order of 1.0 GHz.

10. Apparatus according to claim 4 in which means are included forsupporting both said primary and reference arrays along said same line,said reference array being spaced a predetermined distance along saidsame line from one end radiating element of said primary array.

1. In a radio navigation beacon system which simulates motion byelectrical sequencing of antenna elements to generate an apparentDoppler frequency component at a receiving station, the combinationcomprising: a first source of radio frequency energy of a firstfrequency; a first antenna array including a plurality of equally spacedantenna radiating elements disposed in a line at an angle with respectto the ideal navigational path; first commutating means for successivelyswitching the output of said first source to each of said radiatingelements of said first array according to a predetermined pattern; asecond antenna array including a plurality of equally spaced antennaradiating elements disposed in a line at an angle with respect to saidideal navigational path, said second array having an overall lengthequal to the spacing between adjacent elements of said first array; asecond source of radio frequency energy of a second frequency differentfrom said frequency; and second commutating means for successivelyswitching the output of said second source of radio frequency energy toeach of said radiating elements of said second array in successionduring the time of energization of each of said first array elements bysaid energy of said first frequency.
 2. Apparatus according to claim 1in which the elements of said first and second arrays are disposedsubstantially on the same line.
 3. Apparatus according to claim 2 inwhich said second commutating means is adapted to switch said energy ofsaid second frequency successively among said elements of said secondarray in the opposite direction along said same line as the direction ofswitching provided by said first commutating means at any time. 4.Apparatus according to claim 3 in which said angle of said same linealong which said first and second arrays are disposed makes an angle ofsubstantially 90* with respect to said ideal navigational path. 5.Apparatus according to claim 3 in which said first array is identifiedas the primary array and said second array is identified as thereference array, in which said primary array is defined as having anoverall length which is an integral number of wavelength times theoverall length of said reference array.
 6. Apparatus according to claim5 in which the elements of said primary array are spaced fourwavelengths apart and the elements of said reference array are spacedone-third wavelength apart, whereby said first and second commutatingmeans operate to produce discrete radiated energy path length changes insteps of one-third wavelength.
 7. Apparatus according to claim 6 inwhich said primary array comprises 30 elements and said reference arraycomprises 12 elements.
 8. Apparatus according to claim 5 in which saidsecond frequency is defined as being different from said first frequencyby an amount comparable to the Doppler frequencies produced by saidcommutators, said difference between said first and second frequenciesbeing a small percentage of said first or second frequency.
 9. Apparatusaccording to claim 8 in which said frequency difference is on the orderof 20 KHz, whereas said first and second frequencies are on the order of1.0 GHz.
 10. Apparatus according to claim 4 in which means are includedfor supporting both said primary and reference arrays along said sameLine, said reference array being spaced a predetermined distance alongsaid same line from one end radiating element of said primary array.